Because it is frequently desirable to increase the load carrying capacity of a vehicle without increasing the maximum permissible load on each wheel and tire assembly of the vehicle as regulated by state law, auxiliary axle assemblies have been provided. A number of different auxiliary axle assemblies are presently available for the mounting on such vehicles. Usually, these auxiliary axle assemblies have the capability of being lifted from the road when the maximum permissible load for each wheel and tire assembly already provided on the vehicle is not exceeded. These liftable auxiliary axles are usually provided on various kinds of trucks where such trucks travel for a significant portion of the time during their use in an unloaded condition with the auxiliary axle lifted from the road. One type liftable auxiliary axle assembly is illustrated in U.S. Pat. Nos. 2,742,302; 3,133,735; 3,689,102; 3,713,663; and 3,747,948. A different type of liftable auxiliary axle assembly is illustrated in U.S. Pat. Nos. 3,502,348; 3,694;001; and 3,704,896. A number of different mechanisms for lifting these auxiliary axles have been proposed. One lifting mechanism has been the air bag springs normally used to spring the auxiliary axle where the deflation of the air bag is used to lift the axle. Another proposal is set forth in U.S. Pat. No. 3,325,181 which uses a separate air bag to lift the auxiliary axle. Yet another proposed mechanism is a fluid cylinder attached directly to the auxiliary axle in the frame on the vehicle such as that proposed in U.S. Pat. No. 3,704,896 while another proposal which uses cables in connection with a fluid cylinder is shown in U.S. Pat. No. 2,712,856. These lifting mechanisms suffer from several problems. One of these problems is that the lifting mechanism which must move with the auxiliary axle is relatively heavy making the axle difficult to spring when it is in its lowered road engaging position. Another of these problems is that the lifting mechanism is subject to failure and is expensive to replace. Another of the problems associated with the lifting device is that they are difficult to maintain in adjustment.
It is also frequently desirable to mount the auxiliary axle assembly rearwardly of the drive axles of the truck on which they are mounted as is illustrated in U.S. Pat. No. 3,704,896, especially trucks such as concrete mixers. One of the problems with mounting said auxiliary axle assembly rearwardly of the drive axles of the truck is that the moment applied to the truck chassis by the auxiliary axle assembly when it is in use frequently causes overloading of the front tires of the truck.